1. Field of the Invention
The present invention relates to an electromagnetic shielding sheet that is placed on the observation side of an electromagnetic device such as a display to shield electromagnetic waves generated by the electromagnetic device, and through that the electromagnetic device can be seen. More particularly, the present invention relates to an electromagnetic shielding sheet composed of a net like metallic foil and a transparent substrate film that are laminated with an adhesive layer, and to a method for producing the same.
2. Description of Related Art
Electromagnetic waves generated by electromagnetic devices interfere with the normal functioning of other electromagnetic devices, and are also considered to adversely affect the human body and animals. In particular, plasma display panels (hereinafter referred simply to as xe2x80x9cPDPsxe2x80x9d), which have recently been put to practical use, produce electromagnetic waves with high frequencies of 30 to 130 MHz, so that they can affect computers or computerized devices located near the PDPs. It is therefore desirable to shield such electromagnetic waves as much as possible.
To shield electromagnetic waves, a variety of measures have already been taken. Proposed so far as an electromagnetic shielding means that can meet two essential requirements, electromagnetic shielding ability and visibility, are electromagnetic shielding sheets having both transparency and electro-conductivity, obtainable by forming transparent indium tin oxide (ITO) films on transparent substrate films (see Japanese Laid-Open Patent Publications No. 278800/1989 and No. 323101/1993, for example). It is, however, impossible to fully ensure electro-conductivity with such electromagnetic shielding sheets.
In view of the above drawback, electromagnetic shielding sheets, which are laminates of transparent substrate films and netlike metallic foils obtained by etching, have recently begun to be used (see Japanese Laid-Open Patent Publications No. 119675/1999 and No. 210988/2001, for example). These electromagnetic shielding sheets can satisfactorily shield even electromagnetic waves of high intensities generated by such electromagnetic devices as PDPs, if the thickness of the metallic foils and the size of the meshes of the nets are appropriately controlled. Moreover, these electromagnetic shielding sheets have sufficiently high transparency, so that images displayed on displays covered with them can be clearly seen through them.
Incidentally, a conventional process of producing an electromagnetic shielding sheet of the latter type is as follows: after laminating a metallic foil and a transparent substrate film with an adhesive layer, the metallic foil is made into net form by a photolithographic process.
In the production of an electromagnetic shielding sheet of the latter type, however, fine air bubbles tend to be incorporated into the adhesive layer while the metallic foil and the transparent substrate film are laminated.
To our knowledge, the generation of such air bubbles is mainly attributed to the roughened surfaces of the metallic foil, especially the one on which the adhesive layer will be formed (adhesive-layer-side surface), and air bubbles are more readily produced when the roughness of the adhesive-layer-side surface of the metallic foil is higher. The air bubbles thus incorporated into the adhesive layer not only impair the adhering properties of the adhesive layer but also irregularly reflect light to increase reflectance. An electromagnetic shielding sheet having an adhesive layer containing air bubbles can thus lower the image contrast when placed on a display such as a PDP.
The present invention has been accomplished in order to overcome the above drawback in the related art. An object of the present invention is therefore to provide an electromagnetic shielding sheet that can be obtained while preventing, in order to avoid increase in reflectance, incorporation of air bubbles in an adhesive layer during lamination of a metallic foil and a transparent substrate film with the adhesive layer. Another object of the present invention is to provide a method of producing such an electromagnetic shielding sheet.
To attain the above objects, the adhesive-layer-side surface of the metallic foil is specified, as will be described later in detail, to have such roughness that it can be smooth like mirror surface or the like. In general, a metallic foil has been processed so that its both surfaces can have almost the same smoothness. This means that if the adhesive-layer-side surface of the metallic foil is smooth, the other surface of the metallic foil is smooth as well. To make a metallic foil into net form, a photosensitive resin layer useful for etching is usually formed on the surface of the metallic foil, opposite to the adhesive-layer-side surface, and pattern-wise exposure is conducted with the use of a photomask in sheet form, which is placed on the surface of the photosensitive resin layer. To successfully conduct this pattern-wise exposure, it is necessary to bring the photosensitive resin layer and the photomask in sheet form into close contact, and, for this purpose, vacuuming is conducted before effecting pattern-wise exposure. If the surface of the metallic foil, opposite to the adhesive-layer-side surface, is smooth, the surface of the photo sensitive resin layer formed on this surface also becomes smooth. It is therefore not easy to exhaust, by vacuuming, the air confined between the smooth surface of the photosensitive resin layer and the photomask, and longer time is thus required for vacuuming.
A further object of the present invention is therefore to provide an electromagnetic shielding sheet capable of eliminating the above-described problem, that is, increase in time required for vacuuming needed for making a metallic foil into net form. A still further object of the present invention is to provide a method of producing such an electromagnetic shielding sheet.
An electromagnetic shielding sheet according to the present invention includes a transparent substrate film; and a netlike metallic foil with a densely spaced array of openings, laminated to one surface of the transparent substrate film with an adhesive layer; wherein at least the adhesive-layer-side surface of the metallic foil has such surface roughness that the maximum height Rmax defined in JIS (Japanese Industrial Standards) B0601 is more than 0 and less than 4 xcexcm.
According to the electromagnetic shielding sheet of the present invention, the adhesive-layer-side surface of the metallic foil is specified to have such roughness that the upper limit of the maximum height Rmax is 4 xcexcm, so that the adhesive layer never contains air bubbles in sizes unfavorable from the viewpoint of reflectance. Therefore, when this electromagnetic shielding sheet is placed on a display, the image contrast is scarcely lowered, and the image visibility is maintained high.
In the electromagnetic shielding sheet according to the present invention, it is preferable that the maximum height Rmax of the adhesive-layer-side surface of the metallic foil be more than 0 and 2 xcexcm or less. As long as the maximum height Rmax falls in this range, even if the adhesive layer is of such a type that air bubbles incorporated into it grow during aging, it never contains, after aging, air bubbles that have grown to such sizes that reflectance is adversely affected.
Further, in the electromagnetic shielding sheet according to the present invention, the surface of the metallic foil, opposite to the adhesive-layer-side surface, has such surface roughness that the arithmetic mean roughness Ra defined in JIS B0601 is in the range between 0.02 xcexcm and 1 xcexcm. Since the surface of the metallic foil, opposite to the adhesive-layer-side surface, is specified to have such roughness that the lower limit of the arithmetic mean roughness Ra is 0.02 xcexcm, it is easy to exhaust the air present between a photosensitive resin layer formed on this surface of the metallic foil and a photomask in sheet form placed on the photosensitive resin layer by vacuuming conducted before effecting pattern-wise exposure of the photosensitive resin layer. Operating efficiency is thus improved.
In addition, it is preferable that, in the electromagnetic shielding sheet according to the present invention, a surface subjected to darkening treatment is formed on the adhesive-layer-side surface of the metallic foil. It is also preferable that a surface subjected to darkening treatment is formed on the surface of the metallic foil, opposite to the adhesive-layer-side surface, wherein the reflectance of visible light at the surface subjected to darkening treatment is 5% or less. Since a surface subjected to darkening treatment, i.e., a darkening layer is formed on the adhesive-layer-side surface or both surfaces of the metallic foil, and the surface subjected to darkening treatment is specified so that the reflectance of visible light at the surface subjected to darkening treatment is 5% or less, the lowering of image contrast, which is usually caused when an electromagnetic shielding sheet is placed on a display, can more effectively be prevented.
In the electromagnetic shielding sheet according to the present invention, the adhesive layer can contain air bubbles and the diameters of these air bubbles are preferably less than 50 xcexcm when reflectance is taken into consideration. Further, if the growth of air bubbles during aging is also taken into account, it is more preferable to control the diameters of air bubbles incorporated into the adhesive layer to 20 xcexcm or less so that the air bubbles after aging will never adversely affect the reflectance.
A method of producing an electromagnetic shielding sheet according to the present invention includes the steps of making, as the base of an electromagnetic shielding sheet, a laminate by laminating a metallic foil to one surface of a transparent substrate film with an adhesive layer; laminating a photosensitive resin layer to the metallic foil in the laminate; conducting pattern-wise exposure by applying ionizing radiation to the photosensitive resin layer formed on the metallic foil in the laminate; developing the exposed photosensitive resin layer so that a resist layer can remain in the predetermined pattern; and etching the metallic foil in the laminate by making use of the resist layer to make the metallic foil into net form with a densely spaced array of openings; wherein at least the adhesive-layer-side surface of the metallic foil has such surface roughness that the maximum height Rmax defined in JIS B0601 is more than 0 and less than 4 xcexcm.
According to the method of producing an electromagnetic shielding sheet of the present invention, the adhesive-layer-side surface of the metallic foil to be laminated to the transparent substrate film is specified to have such roughness that the upper limit of its surface roughness (maximum height Rmax) is 4 xcexcm. Therefore, air bubbles in sizes unfavorable from the viewpoint of reflectance are not incorporated into the adhesive layer while the metallic foil and the transparent substrate film are laminated with the adhesive layer. It is thus possible to obtain an electromagnetic shielding sheet that scarcely lowers the image contrast and can maintain image visibility high when placed on a display.
In the method of producing an electromagnetic shielding sheet according to the present invention, it is preferable that the maximum height Rmax of the adhesive-layer-side surface of the metallic foil be more than 0 and 2 xcexcm or less. As long as the maximum height Rmax falls in this range, even if the adhesive layer is of such a type that air bubbles incorporated into it grow during aging, it never contains, after aging, air bubbles that have grown to such sizes that reflectance is adversely affected.
In addition, the method of producing an electromagnetic shielding sheet according to the present invention further includes the step of placing a photomask in sheet form on the photosensitive resin layer and vacuuming to a predetermined pressure to bring the photosensitive resin layer and the photomask into close contact; wherein, in the step of conducting pattern-wise exposure, the ionizing radiation is applied to the photosensitive resin layer through the photomask, and at least the surface of the metallic foil, opposite to the adhesive-layer-side surface, has such surface roughness that the arithmetic mean roughness Ra defined in JIS B0601 is in the range between 0.02 xcexcm and 1 xcexcm. Since the surface of the metallic foil, opposite to the adhesive-layer-side surface, to which the photosensitive resin layer and the photomask in sheet form are laminated in the order mentioned, is specified to have such roughness that the lower limit of the arithmetic mean roughness Ra is 0.02 xcexcm, it is easy to-exhaust the air present between the photosensitive resin layer and the photomask placed thereon by vacuuming conducted before effecting the pattern-wise exposure of the photosensitive resin layer. Operating efficiency is thus improved.
In the method of producing an electromagnetic shielding sheet according to the present invention, the adhesive-layer-side surface or both surfaces of the metallic foil in the laminate may be subjected to darkening treatment. An electromagnetic shielding sheet containing a metallic foil that has been subjected to darkening treatment scarcely lowers the image contrast when placed on a display.